Lutton, "Technical Lipid Structures," J. Amer. Oil Chem. Soc., 49, pp. 1-9 (1972) describes the interconversion of 1,2-diglycerides to 1,3-diglycerides on storage.
U.S. Pat. No. 3,845,087, issued to Degroot (1974) also describes the isomerization of 1,2-diglycerides to 1,3-diglycerides. This isomerization is accomplished in the solid state within 20.degree. C. of the initial melting point of the 1,2-diglyceride. This U.S. patent appears to correspond with British Pat. No. 1,369,438.
Eckey and Formo, J. Amer. Oil Chem. Soc., 26, pp. 207-211 (1949) describe a method of preparing saturated diglycerides or saturated monoglycerides by directed interesterification of triglyceride mixtures containing free hydroxyl groups.
U.S. Pat. No. 2,875,066, issued to Holman and Going (1959) describes a low temperature "directed interesterification" of glycerides. The conditions are such that the desired glyceride is simultaneously crystallized from the reaction.
U.S. Pat. No. 2,558,547, issued to Eckey (1951) describes a catalytic ester-ester interchange at temperatures not substantially exceeding 120.degree.l C. The reaction was carried out using finely divided and suspended alkali metal hydride as the catalyst.
U.S. Pat. No. 3,012,890, issued to Dutton, et al. (1961) relates to a process for preparing a mixture of 1,3-diglycerides by reacting glycerin, tripalmitin, tristearin and triacetin. The random mixture of diglycerides is separated and crystallized.
U.S. Pat. No. 2,206,167, issued to Edeler and Richardson (1940) discloses the preparation of mono- or diglycerides by reacting glycerin, a triglyceride and soap at temperatures above 150.degree. C.
U.S. Pat. No. 3,312,724, issued to Harwood (1967) describes a method for preparing symmetrical 1,3-diglycerides by reacting glycerin with triglycerides and a catalyst in the presence of a solvent.
U.S. Pat. No. 3,492,130, issued to Harwood (1970) is related to hard butter compositions. In Example I of this patent a process for preparing 1,3-dipalmitin from tripalmitin and glycerin is described.
U.S. Pat. No. 3,634,473, issued to Harwood (1972) describes a process for preparing pure symmetrical diglycerides by a glycerolysis process. "The improvement resides in first liquifying a substantially anhydrous mixture of glycerol and triglyceride fat, then adding a low temperature rearrangement catalyst to the liquified mixture and agitating the catalyst-containing liquified mixture until a symmetrical diglyceride is formed by equilibration in the liquid mixture. Crystallization of the symmetrical diglyceride is induced. . . ."
The following references teach the use of catalysts and acid halides for manufacturing 2-oleoyl triglycerides suitable for use as confectioner's butters from 1,3-diglycerides of the type prepared in the manner of this invention.
U.S. Pat. No. 3,809,711, above, discloses the perfluorinated sulfonic acids preferred for use in the esterification step of the present process.
U.S. Pat. Nos. 3,410,881 and 3,337,596 disclose the use of perchloric acid as an effective catalyst for preparing a cocoa butter substitute without rearrangement of the ester groups. However, perchloric acid is known to be explosive and its use in the presence of organic compounds in commercial scale syntheses can be dangerous.
Another known method for synthesizing a cocoa butter substitute comprises reacting a diglyceride having palmitoyl and stearoyl groups at the 1- and 3-positions with oleoyl chloride; see U.S. Pat. No. 3,012,890. Furthermore, it is known in the prior art that, in general, acid chlorides can be used to specifically esterify mono- and diglycerides. The use of acid chlorides for specific esterifications has many undesirable aspects, however. For instance, acid chlorides are very corrosive and their use involves handling problems. Besides, hydrochloric acid, a by-product of the reaction of an acid chloride with a hydroxyl group, is difficult to remove from the oleaginous reaction product, a critical factor inasmuch as the product is to be used as a food.
Feuge, Willich and Guice, the Journal of the American Oil Chemists Society, July 1963, pp. 260-64, demonstrate that ester group rearrangement ordinarily occurs during the esterification of partial glycerides, and, at page 260, point out that hydrochloric, sulfuric and hydrocarbyl sulfonic acids, which are widely used as esterification catalysts, cause ester group rearrangement. Accordingly, these acid catalysts are not suitable for preparing the desired position-specific (i.e., 2-oleoyl) triglycerides for use as a cocoa butter substitute. Similarly, ester group rearrangement ordinarily occurs during esterification of partial polyol esters other than glycerides, e.g., during esterification of partial 1,2-propylene glycol esters.
Gramstad, et al., J. Chem. Soc. 4069, 1957, teach the use of trifluoromethane sulfonic acid as an esterification catalyst but do not suggest that it would be useful in the preparation of triglycerides without ester rearrangement. Moreover, the previously noted Feuge, et al., article suggests that sulfonic acid esterification catalysts per se cause ester group rearrangement.
U.S. Pat. No. 3,882,155 discloses the use of trinitrobenzene sulfonic acid as a position-specific esterification catalyst for synthesizing cocoa butter.
U.S. Pat. No. 3,808,245 discloses boron trifluoride as a position-specific esterification catalyst for synthesizing cocoa butter.
U.S. Pat. No. 3,989,728 discloses metal halides as position-specific catalysts in synthetic cocoa butter processes.
The disclosures of all the foregoing patents and articles are incorporated herein by reference.